TechTopics No. 75 Ferroresonance in ungrounded systems with voltage transformers connected line-to-ground

TechTopics No. 75
Ferroresonance in ungrounded systems with
voltage transformers connected line-to-ground
Ferroresonance can occur when voltage transformer (VT)
primaries are connected line-to-ground in a circuit that is
normally ungrounded or that can become ungrounded during
certain switching operations. When the VTs are connected lineto-ground on an ungrounded system, the VT primary becomes
a principal path for system ground current.
Ferroresonance can occur when a system transient occurs that
causes a ringing current (resonance) between the magnetizing
reactance of the VT and coupling capacitance to ground, under
conditions in which the VT secondary circuit is lightly loaded.
The “IEEE Standards Dictionary: Glossary of Terms &
Definitions” (formerly known as IEEE Std 100), contains this
definition (among others):
“Ferroresonance...A phenomenon usually characterized by
overvoltages and very irregular wave shapes and associated
with the excitation of one or more saturable inductors
through capacitance in series with the inductor.”
This definition elegantly describes the phenomenon, but more
importantly gives an indication of the problem ferroresonance
poses for the system. The overvoltages that result from
ferroresonance will overstress the dielectric capabilities of the
system and also will likely damage the voltage transformers.
Now, some more discussion of what actually happens during a
ferroresonance condition.
The value of the magnetizing reactance of the VT is a function
of the flux in the iron core. The coupling capacitance of each
phase of the system is the value of capacitance between the
primary phase conductor and ground. The magnetizing
reactance and coupling capacitance form a parallel circuit of
capacitance (C) and inductance (L) from line to ground, in
other words, an LC circuit. If the VT operates in its linear
magnetic range, no particular problem exists. However, the LC
circuit requires only a relatively small voltage transient to make
the circuit ring at the resonant frequency of the LC circuit.
Once the circuit begins to ring (resonate), constructive
interference during the oscillations causes the voltage to
increase to high levels. When the voltage reaches a sufficiently
high level, the VT magnetic circuit is driven into saturation,
causing the reactance to collapse. However, the charge
trapped in the capacitance at this high-voltage level is still
significant, and the capacitive charge is discharged through the
(low) saturated VT reactance.
If the VT secondary circuit is lightly loaded, there is very little
resistance (and therefore, losses) and the current through the
VT winding can easily reach levels that will damage the VT
winding before the VT fuse interrupts. (In this respect, it should
be noted that primary fuses for VTs do not protect the VTs from
damage by overcurrent, but are intended to separate the VTs
from the power system so that a complete power system
shutdown is avoided.)
Answers for infrastructure.
The “classic” method to deal with this situation is to add
resistance (R) to the circuit, converting it from a simple LC
circuit to a RLC (damped) circuit. Two methods are commonly
used to avoid problems with ferroresonance:
1. Connect the VTs with primary windings connected in
grounded wye, with secondary windings connected in open
delta, with a loading resistor across the open delta (probably
most common). The resistance value should be chosen so
that dissipation during ground fault conditions is
approximately 50 percent of the continuous VA rating of a
single VT.
2. Connect the VTs with both primary and secondary in wye,
with a loading resistor across the secondary of each VT.
Various references indicate that the resistor dissipation
during ground fault conditions should be in the range of the
excitation watts of the VT, up to 50 percent of the
continuous VA of a single VT. Problems have been reported
with the lower value, and a higher value is recommended.
In either case, VTs must be rated for full line-to-line voltage.
Ferroresonance is an infrequent occurrence, very much
influenced by system topology and conditions. It is also
influenced by the inductance of the VT and the nature of
system transients. Thus, ferroresonance does not happen in
every system. For those applications in which ferroresonance is
a problem, loading resistance in the VT secondary circuit is
normally sufficient to alleviate the affects of ferroresonance.
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